Genetic and epigenetic modifications are critical to all stages of cancer disease progression and epigenetic silencing has been shown to be important in the misregulation of genes involved in all of the hallmarks of cancer (Jones, P. A. and Baylin, S. B. (2007) “The epigenomics of cancer”, Cell, Vol. 128, pp. 683-692). The underlying epigenetic modifications that mediate regulation include DNA methylation and post translational histone modification. The latter includes methylation, acetylation, and ubiquitination. DNA-demethylating agents and histone deacetylase inhibitors have shown anti-tumour activity and a number of agents have been approved for use in the treatment of haematological malignancies. The enzymes mediating histone modification, including histone acetyltransferases (HATs) which acetylate histone and non-histone proteins, represent a wave of second generation targets for small molecule drug intervention.
Prostate cancer is one of the most common malignancies, and the second leading cause of cancer mortality among men. The treatment for clinically localised disease is typically surgery or radiation therapy. For patients who recur systemically after definitive treatment, or who present with loco-regional or metastatic disease, long term disease control is the primary objective. Typically, this entails a series of hormonal therapies that supress androgen receptor (AR) signalling, since prostate cancers are exquisitely dependent upon AR function for survival and progression. Although AR targeted therapies inhibit tumour growth, disease is rarely eliminated and resistance to therapy is acquired through restored AR function. Progression to this ‘castration resistant’ prostate cancer (CRPC) represents the lethal phenotype of the illness. It is estimated that between 50-60% of patients that develop metastatic disease have CRPC. Recently, several new therapeutic agents have been approved for the treatment of CRPC. These however, provide limited clinical efficacy and serve only to prolong progression. Novel and tolerable agents are therefore necessary to make further gains in the treatment of CRPC.
Multiple cellular mechanisms lead to the progression of CRPC. In all cases, acquisition of the CRPC phenotype is mediated via re-activation of the AR pathway. The acetyltransferase p300 directly regulates AR levels and AR signalling activity in prostate cancer cells (Zhong et al., ‘p300 acetyltransferase regulates androgen-receptor degradation and PTEN-deficient prostate tumorigenesis,’ Cancer Res., Vol. 74, pp. 1870-1880, 2014). Therapeutic modulation of p300 activity would therefore target all known adaptive mechanisms which lead to the development of CRPC. Approved therapies and those in clinical studies primarily target only one or other of theses cellular mechanisms. The modulation of p300 activity directly provides an opportunity to more broadly modulate AR activity in CRPC than current and other experimental therapeutic strategies. In addition, resistance mechanisms to recently approved agents have been shown to be AR-dependent (Cai, C. et al., (2011) ‘Intratumoral de novo steroid synthesis activates androgen receptor in castration-resistant prostate cancer and is up-regulated by treatment with Cyp17A1 inhibitors,’ Cancer Res., Vol. 71, pp. 6503-6513). Modulation of p300 should therefore inhibit resistance to current therapies and potentially provide improved and sustained efficacy and greater clinical utility.
In common with p300, the CREB (cyclic-AMP response element binding protein) binding protein (CBP) is an acetyltransferase that acts as a transcriptional co-activator in human cells. Both CBP and p300 possess a single bromodomain (BRD) and a lysine acetyltransferase (KAT) domain, which are involved in the post-translational modification and recruitment of histones and non-histone proteins. There is high sequence similarity between CBP and p300 in the conserved functional domains (see Duncan A. Hay et al, JACS 2014, 135, 9308-9319). Modulation of CBP activity therefore provides a promising route to the treatment of certain cancers. Accordingly, compounds that can modulate, e.g. inhibit, the activity of p300 and/or CBP are of interest in cancer therapy.
Tumours which harbour loss of function mutations in CBP become addicted to p300 and are uniquely sensitive to p300 inhibition (see Ogiwara et al. 2016 Cancer Discovery. 6; 430-445). Conversely tumours with mutations in p300 are uniquely sensitive to CBP inhibition. Genetic analysis reveals that up to 15% of both non-small cell and small cell lung tumours have these loss of function mutations. Similar mutations are also found in up to 25% of bladder cancers. Accordingly, compounds that can modulate, eg inhibit, the activity of p300 and/or CBP are of interest in cancer therapy for tumours with these molecular changes.
Furthermore, CBP/p300 regulates the expression of key tumour immune checkpoint proteins such as CTLA4/PD-L1 (see Casey et al., Science. 352; p 227-231, 2016) and plays an important role in the differentiation and function of T-regulatory cells which are involved in immune evasion by tumours. Accordingly, compounds that can modulate, eg inhibit, the activity of p300 and/or CBP are of interest for cancer therapy in combination with agents that target the onco-immune system.